The presently disclosed subject matter relates to an electrocardiogram analyzer and electrode set in which the influence of noise caused by the cardiopulmonary resuscitation (hereinafter, abbreviated as CPR) can be reduced.
As a life-saving measure for a patient in cardiac arrest, it is important to constantly perform chest compression. For a cardiac arrest patient who experiences ventricular fibrillation or pulseless ventricular tachycardia, delivery of electric shock constitutes an important factor. In order to rapidly perform adequate procedures on such a patient, therefore, an analytically accurate measurement of an electrocardiogram is necessary even during chest compression. When chest compression is performed on a patient in cardiac arrest, however, noise contaminates an electrocardiogram, and impedes analyzing of the electrocardiogram.
Non-patent Document 1 below reports that the noise contamination is not caused by deformation of the heart or a change of the chest impedance due to the compression, but by a disturbance which is applied between an electrode and the skin. Moreover, a disturbance which is applied between an electrode and the skin during chest compression changes in various factors such as the strength and rate of the compression, characteristics of the electrode, and the condition of the portion to which the electrode is bonded (the muscle and fat masses, and the dry condition of the skin). Therefore, noise appears unevenly in an electrocardiogram, and hence it is difficult to obtain an adequate analysis result from an electrocardiogram contaminated with noise.
Therefore, Patent Document 1 below proposes a technique in which features in the frequency region of single or plural electrocardiogram leads are analyzed, thereby determining whether electric shock is necessary or not, irrespective of the presence or absence of noise caused by chest compression.
Patent Document 2 below discloses a filtering technique which is based on detection of pulses (beats of the heart) contained in an electrocardiogram. In this technique, a baseline variation between pulses is captured, and the cutoff frequency band of a low-cut filter is changed, thereby removing noise. This technique can be applied also to noise contamination due to chest compression.    (Patent Document 1) JP-A-2011-189139    (Patent Document 2) Japanese Patent No. 4,830,857    (Non-patent Document 1) Determination of the noise source in the electrocardiogram during cardiopulmonary resuscitation, Crit Care Med 2002 Vil. 30, No. 4
In noise caused by chest compression, however, peaks appear not only at the frequency corresponding to the rate of the chest compression, but also at frequencies which are integer multiples of the frequency. Therefore, there is a possibility that the determination may not be correctly performed by the technique disclosed in Patent Document 1. For example, the rate of chest compression which is recommended in the guidelines is 100 times/minute. In this case, peaks appear at the fundamental period of 1.67 Hz, and also at integer multiple frequencies such as 3.33 Hz which is twice the period, and 5.0 Hz which is thrice the period. On the other hand, the frequency of ventricular fibrillation is about 2 to 10 Hz. The above-mentioned twice and thrice frequencies are within the range. Even when the fundamental period is removed by a low-cut filter, therefore, there is a possibility that peaks appearing at integer multiples of the fundamental period may affect the result of feature analysis.
The technique disclosed in Patent Document 2 has an advantage that the cut-off band can be changed in accordance with contaminating noise. However, there is a possibility that, when the cut-off band of the low-cut filter is set to be extremely high, even components of an electrocardiogram may be removed. Moreover, the technique is based on detection of the QRS wave indicating the beat of the heart. Therefore, the filtering process does not properly function in the case of ventricular fibrillation or cardiac arrest in which a beat does not exist.